As a typical permanent magnet manufactured at the present, there are known Alnico magnets, ferrite magnets, rare earth metal magnets and the like. The Alnico magnets have been manufactured for a very long time, but their demand is lowering in accordance with the development of cheap ferrite magnets and rare earth metal magnets having higher magnetic properties. On the other hand, the ferrite magnets are chemically stable and low in cost because oxides are used as a main starting material, so that they are the main source of magnetic material even at the present. However, ferrite magnets have a drawback in that the maximum energy product is small.
Recently, Sm--Co series magnets having a combination of magnetic isotropy inherent to rare earth metal ions and magnetic moment inherent to transition metal elements have been developed, whereby the conventional value of maximum energy product is largely increased. However, the Sm--Co series magnet is mainly composed of resourceless Sm and Co, so that it is obliged to become expensive.
Now, it has been attempted to develop cheap magnet alloys which do not contain expensive Sm and Co and have high magnetic properties. Consequently, Egawa et al. has developed stable ternary alloys by a sintering process (Japanese Patent Application Publication No. 61-34242 and Japanese Patent laid open No. 59-132104) and J. J. Groat et al. have developed alloys having a high coercive force by a liquid quenching process (Japanese Patent laid open No. 59-64739). These magnets are composed of Nd, Fe and B, and their maximum energy product exceeds that of Sm--Co series magnets.
However, Nd--Fe--B series magnets contain greater amounts of a light rare earth element such as Nd having very high activity or the like and corrosive Fe as a main component, so that the corrosion resistance is poor and hence the magnetic properties and reliability as an industrial material are degraded.
Therefore, in order to improve the corrosion resistance, countermeasures have been taken, such as surface plating (Japanese Patent laid open No. 63-77103), coating treatment (Japanese Patent laid open No. 60-63901) and the like on the sintered magnets, and surface treatment on resin bonded type magnets before kneading magnet powder with a resin and the like. However, such countermeasures can not be said to be an effective rustproof treatment over a long period of time, and the manufacturing cost becomes higher due to such a treatment. Further, such treatment results in problems such as magnetic flux loss due to the presence of the protective film and the like.
As a solution to the above problems, the inventors have previously proposed rare earth metal-transition metal-boron series magnet alloys in which Fe in the Nd--Fe--B series magnet is replaced with high concentrations of Co and Ni (Japanese Patent laid open No. 2-4939).
Such magnets are excellent in corrosion resistance and high in Curie point, so that the reliability as a magnet material is largely increased.
The present invention is concerned with rare earth metal-transition metal series magnets of two phase structure further developed from the above magnet.
Moreover, magnets having excellent magnetic properties through two alloying processes in which rare earth rich phase and rare earth poor phase are mixed and sintered at liquid phase state have previously been proposed as Nd series magnets of two phase structure (Japanese Patent laid open No. 63-93841 and No. 63-164403). In this case, the magnetic properties are improved, but there still remains the problem of corrosion resistance.